Low light level, high resolution imager using phosphor screen provided with a metal layer for controlling integration cycle of photosensitive matrix array

ABSTRACT

The present invention relates to a low level, high resolution imager of the type comprising a light-amplifier tube (300) including a photocathode (310), at least one microchannel slab (330) serving as an electron amplifier, and a light-emitting phosphor screen (340) provided with a metal layer (342), an electron camera (400) comprising a photosensitive matrix array (410) suitable for transforming a received photon into an electron, and control means (700) for the electron camera, characterized by the fact that the control means (700) comprise an amplifier (710) responsive to the electrons collected on the metal layer (342) of the light-emitting phosphor screen (340) to control integration cycles of the photosensitive matrix array (410) in repetitive one-shot mode synchronized on the appearance of photons at the input of the light-amplifying tube (300).

The invention relates to a low light level, high resolution imager.

A large amount of work in physics (nuclear physics, astrophysics,biophysics) requires it to be possible to locate with great accuracylight sources that are of very low level in terms of photon count.

For example, much work performed in molecular biology derives itsinformation by studying the three-dimensional positioning of chemicalspecies (strands of DNA or of RNA). To do this, the commonly-usedtechnique consists in marking the species being studied with a specificradioactive probe. An experimental result is thus expressed in the formof a map of β-emitting populations, the map being optionallyquantitative and of greater or lesser resolution.

For several years, proposals have been made to pick up an image of lightsources using charge coupled devices (CCD).

For example, in the article by Y. Charon et al. entitled "A highresolution β-detector" published in the document Nuclear Instruments andMethods in Physics Research, A 273 (1988), 748-753, a system isdescribed as shown diagrammatically in accompanying FIG. 1, which systemis particularly adapted to experiments in molecular biology, andcomprises:

a sample carrier 10;

a thin scintillator 20 (10 μ);

a light-amplifying tube 30 essentially comprising:

a photocathode 31;

focusing electrodes 32;

a slab 33 of microchannels that amplify electrons by secondary emission;and

a light emitting phosphor screen 34; an electron camera 40 comprising:

a charge coupled device (CCD) 41;

a control module 42; and

a shaping module 43;

a governing computer 50;

an external trigger generator 60; and

an interface card 70 between the generator 60, the camera 40, and thecomputer 50.

The scintillator 20 generates photons when it detects an electron comingfrom the sample or from an equivalent source. The light is amplified inthe tube 30 and is then applied to the electron camera.

The module 42 controls this camera in one-shot mode, not in video mode.In video control mode, frame-cycles follow one another at a fixed rate(where each cycle is made up of a stage during which the charge coupleddevice is reset to zero, followed by an image integration stage, andthen by a read stage). In contrast, in one-shot control mode, eachframe-cycle is controlled independently of the preceding cycle.

More precisely, according to the above-mentioned document, the camera 40is controlled in repetitive one-shot mode by the external triggergenerator 60, i.e. the camera 40 is controlled to have short andrepetitive integration cycles as opposed to a simple one-shot mode whichwould consist in integrating the image of the light source over a longperiod and then in reading it solely at the end of acquisition.

The control of the camera 40 is shown diagrammatically in accompanyingFIGS. 2, 3, and 4.

FIG. 2 shows the time distribution of a light source or sample.

FIG. 3 shows the corresponding response of a light-amplifying tube 30.Noise pulses can be seen in FIG. 3.

Finally, FIG. 4 shows the response of the tube 30 superposed firstly onthe cycles of the charge coupled device 41, each of which includes astage during which the CCD is reset to zero, an image integration stage,and a CCD reading stage, and secondly the signal which triggers thecycles.

The technique of controlling the camera 30 in repetitive one-shot modemakes it possible to escape in part from the major cooling required inrepetitive one-shot mode because of the contribution of thermal noise inthe light-amplifying tube and in the camera, which noise is proportionalto the integration time.

However, controlling the camera 30 in competitive one-shot mode does notgive complete satisfaction. It suffers from the following drawbacks:

events are lost during dead time, thereby loosing efficiency;

given the integration time, the contribution of thermal noise from theamplifier tube remains large; and

pixel brightness information cannot be used quantitatively since eventsoccur randomly within the integration window.

Attempts have been made to eliminate these drawbacks by triggering theintegration cycle of the charge coupled device only in the presence of alight event.

The system proposed to do this is described in the article by Y. Charone1. entitled "H.R.R.I., a high resolution β- imager for biologicalapplications", published in Nuclear Instruments and Methods in PhysicsResearch A 292 (1990), 179-186. That system is also showndiagrammatically in accompanying FIG. 5.

There can be seen again in FIG. 5:

the sample carrier 10;

the scintillator 20;

the light-amplifying tube 30;

the electron camera 40;

the governing computer 50; and

the interface card 70.

However, in the system shown in FIG. 5, the external trigger generator60 is replaced by a photomultiplier 80 which is associated with ashaping card 81.

Relative to the sample carrier 10, the photomultiplier 80 is disposed onthe opposite side to the scintillator 20. Thus, the photomultiplierpicks up a fraction of the photons generated by the scintillator 20after they have passed through the sample and the sample carrier 10, forthe purpose of generating a trigger pulse that is synchronized on theappearance of a light event.

The integration time may be adjusted to a minimum value that is afunction solely of the phosphor decay time of the light-amplifier tube30 and of the time required by the reset to zero stage of the chargecoupled device 41.

The cycles obtained in this way, and the corresponding trigger signalsare shown diagrammatically in accompanying FIG. 6.

A comparison of FIGS. 4 and 6 shows that the system in which triggeringis synchronized on the appearance of a light event, as shown in FIG. 5,provides the following advantages:

considerably increased detection efficiency;

a great reduction in the contribution of thermal noise since theintegration time is reduced, while the noise occurs randomly within theintegration window; and

brightness intensity is accurately reproduced, thus making it possibleto perform screening and center-of-gravity processing.

Nevertheless, the system shown in FIG. 5 does not give full satisfactioneither.

That system is firstly dependent on the thickness of the sample used. Ifthe sample is too thick, the photo-multiplier 80 receives little or nolight.

Further, that system is essentially limited to the field ofexperimentation in molecular biology, and it is not suitable, forexample, for use in astrophysics.

An object of the present invention is to improve the situation byeliminating the drawbacks of the prior art.

According to the present invention, that object is achieved by a lowlight level, high resolution imager of the type comprising:

a light-amplifying tube comprising:

a photocathode;

at least one microchannel slab serving as an electron amplifier; and

a light emitting phosphor screen provided with a metal layer:

an electron camera comprising a photosensitive matrix array suitable fortransforming a received photon into an electron; and

control means for controlling the electron camera;

the imager being characterized by the fact that the control meanscomprise an amplifier responsive to the electrons collected on the metallayer of the lightemitting phosphor screen to control integration cyclesof the photosensitive matrix array in repetitive one-shot modesynchronized on the appearance of photons at the inlet to thelight-amplifying tube.

Other characteristics, objects, and advantages of the present inventionappear on reading the following detailed description made with referenceto the accompanying drawings which are given by way of nonlimitingexample, and in which:

FIG. 1, described above, is a diagram of a first previously knownsystem;

FIG. 2 shows the time distribution of a light source;

FIG. 3 shows the corresponding response as collected at the outlet of alight-amplifying tube;

FIG. 4 shows the cycles and the trigger signal of the system shown inFIG. 1;

FIG. 5, described above, is a diagram of a second previously knownsystem;

FIG. 6 shows the cycles and the trigger signal of the system shown inFIG. 5; and

FIG. 7 is a block diagram of an imager of the present invention.

The imager of the present invention shown in accompanying FIG. 7comprises a light-amplifying tube 300, an electron camera 400, a controlcircuit 700, and a computer 500.

The light-amplifying tube 300 is preferably of the proximity focusingtype fitted with two microchannel slabs giving it high gain.

As shown in accompanying FIG. 7, the tube 300 essentially comprises: aphotocathode 310, two slabs 330 and 331 of microchannels that serve aselectron amplifiers, and a phosphor screen 340 constituting an anode.

The phosphor screen 340 comprises, more precisely, a phosphor layer 341covered on its side facing the slabs 330, 331 with a thin layer of metal342, generally of aluminum.

Thus, the shower of secondary electrons corresponding to onephoto-electron being amplified by the slabs 330 and 331 is acceleratedtowards the screen 340. When the electrons are slowed in said screen,light is produced by the excited medium 341 and the electrons arecollected in a few ns on the metal-coated face 342 of the screen.

The electron/electron gain of a tube 300 having two slabs 330, 331 istypically of the order of 105

As mentioned above as constituting an essential characteristic of thepresent invention, the control circuit 700 comprises an amplifier 710responsive to the electrons collected by the metal layer of the screen340 for controlling the integration cycles of the camera 400 via a gate714. The function of the gate 714 is to transform the analog signal fromthe amplifier 710 into a logic signal. The gate 714 operates essentiallyby integration and by comparison with a threshold. For example it may beconstituted by the integrating linear gate sold by the firm SEPH. Thegate 714 is placed between the output of the amplifier 710 and the inputof the module 420.

More precisely, in a preferred embodiment shown in FIG. 7, the metallayer 342 of the screen is connected to ground via a resistor R712 andthe metal layer 342 is connected to a first input of the operationalamplifier 710, while the second input thereof is grounded.

Charge flowing through the resistor R712 thus serves to generate apotential difference at the input of the amplifier 710.

This voltage is amplified by the voltage amplifier 710. The amplifier isof the wide passband and low noise type. The signal is then chargeintegrated and is then subjected to a voltage threshold in the gate 714,such that when the gate is enabled, a trigger signal is applied to themodule 420.

The electron camera 400 used in the context of the present inventionadvantageously comprises a charge coupled device (CCD) 410, a controlmodule 420, and a module 430 for shaping the signals picked up by theCCD, in a manner similar to systems known in the past, and describedabove with reference to FIGS. 1 and 5.

The trigger signal from the gate 714 is then applied to the input of thecontrol module 420 so that each trigger signal initiates a reset-to-zeroor "cleaning" cycle of the CCD, integration of the image on the CCD, andthen reading thereof via the module 430.

The signals obtained in this way then pass via an interface card 720before being directed to the computer 500 where they are processed in amanner that is known per se, as described in the prior documentsdescribed above.

It may be observed that the phosphor screen 340 must have a period thatis compatible with the time taken to reset the CCD 410 to zero. Thescreen is required to store the image during the reset-to-zero time ofthe CCD preceding each integration.

In order to obtain a reset-to-zero duration that is very short, e.g.about 1 μs, it is possible to use an anti-dazzle system of certain CCDsin accordance with the dispositions described in the document by R.E.Ansorge et al., entitled "The UA2 scintillating fiber detector",published in Nuclear Instruments and Methods A 273 (1988), 748-753. Theintegration time can then be reduced proportionally by using a screenhaving a semifast phosphor (a few μs). This type of operation makes thecontribution of the detector's thermal background noise negligible.

The imager of the present invention makes it possible to provide animage of a very low level light source (single photo-electronsensitivity) with a resolution of the order of 20 μm.

It is recalled that a charge coupled device (CCD) is a matrix array ofabout 10⁴ photocells of small size (about 20 μm by 20 μm), each suitablefor transforming a received photon into an electron. During theintegration stage, each cell accumulates a quantity of chargeproportional to the light it receives. Reading consists in sequentiallytransferring the contents of each cell to an imaging device (in thiscase preferably the computer 500 via the interface card 720).

Where applicable, in the context of the present invention, the chargetransfer device 410 may be replaced by a CID type device known to theperson skilled in the art and in which the charge accumulated in eachcell is read directly without transfer.

The inventors have performed tests using an imager comprising alight-amplifier tube 300 with proximity focusing and fitted with twomicrochannel slabs 330 and 331 to obtain an electron/electron gain ofthe order of 10⁵, together with a fast phosphor screen (P47), a CCDelectron camera 400, a low noise (<5 mV) and wide passband (about 200MHz) voltage amplifier 710 having a voltage gain of 100, and anintegrating linear gate 714 as sold by the firm SEPH.

Those tests have shown a triggering efficiency of 90% on incident lightevents of minimum amplitude, i.e. corresponding to singlephotoelectrons. Said efficiency is given by the ratio of the number ofsingle photoelectrons emitted by the photocathode of the tube 300 andthe number thereof actually detected by the imaging system.

The above described imager is designed to detect incident light photons.

Nevertheless, the imager could easily be adapted to detect other typesof incident ray, for example β- rays, merely by placing a system forconverting said incident rays into light, e.g. a scintillator 200,upstream from the tube 300, and as shown in chain-dotted lines in FIG.7.

Naturally, the present invention is not limited to the embodimentsdescribed above, but extends to any variant coming within the spiritthereof.

Where appropriate, it is possible to consider using a tube having asingle microchannel slab for example.

Likewise, it is possible to envisage using an electrostatically focusedlight-amplifying tube as shown diagrammatically in FIGS. 1 and 5.

What is claimed is:
 1. A low light level, high resolution imager of thetype comprising:a light-amplifying tube (300) comprising:a photocathode(310); at least one microchannel slab (330, 331) serving as an electronamplifier; and a light emitting phosphor screen (340) provided with ametal layer (342); an electron camera (400) comprising a photo-sensitivematrix array (410) suitable for transforming a received photon into anelectron; and control means (700) for controlling the electron camera(400); the imager being characterized by the fact that the control means(700) comprise an amplifier (710) responsive to the electrons collectedon the metal layer (342) of the light-emitting phosphor screen (340) tocontrol integration cycles of the photosensitive matrix array (410) inrepetitive one-shot mode synchronized on the appearance of photons atthe inlet to the light-amplifying tube (300).
 2. An imager according toclaim 1, characterized by the fact that the light-amplifying tube (300)includes two microchannel slabs (330, 331).
 3. An imager according toclaim 1 or 2, characterized by the fact that the electron/electron gainof the light-amplifying tube (300) is of the order of
 105. 4. An imageraccording to any one of claims 1 to 3, characterized by the fact thatthe photosensitive matrix array (410) of the electron camera (400) is acharge coupled device.
 5. An imager according to any one of claims 1 to4, characterized by the fact that the metal layer (342) of the phosphorscreen (340) is grounded via a resistor (R712) and is also connected tothe input of the control amplifier (710).
 6. An imager according to anyone of claims 1 to 5, characterized by the fact that a logic shapinggate (714) is interposed between the output of the control amplifier(710) and the electron camera (400).
 7. An imager according to any oneof claims 1 to 6, characterized by the fact that a system (200) suitablefor transforming incident radiation into light is placed upstream fromthe light-amplifying tube (300).
 8. An imager according to claim 7,characterized by the fact that a scintillator (200) is placed upstreamfrom the light-amplifying tube (300).
 9. An imager according to any oneof claims 1 to 8, characterized by the fact that the light-amplifyingtube (300) is a proximity focusing tube.
 10. An imager according to anyone of claims 1 to 8, characterized by the fact that thelight-amplifying tube (300) is a tube with electrostatic focusing. 11.An imager according to any one of claims 1 to 10, characterized by thefact that the metal layer (342) of the light-amplifying tube (300) isbased on aluminum.
 12. An imager according to claim 1, characterized bythe fact that the photosensitive matrix array (410) is of the CID type.